The ability to non-invasively image the molecular composition of an object is desirable in a number of application areas, such as medical imaging, security, structural integrity verification, and homeland defense. While x-ray imaging is the most effective strategy for tomographic imaging in such applications, conventional x-ray systems are not sensitive to molecular composition. X-rays interact with materials via photoelectric absorption, Compton scatter, coherent (Bragg) scatter and fluorescence. Conventional x-ray imaging primarily measures absorption and Compton scatter; however, only coherent scatter and fluorescence are sensitive to molecular or atomic identity.
A traditional x-ray imaging system detects hard and soft materials by the variation in x-ray transmission through an object. More recently, however, imaging systems based on x-ray scattering by the structure of an object have been developed, such as described in U.S. Pat. No. 7,835,495 entitled “System and Method for X-ray Diffraction Imaging,” which discloses a method for x-ray scatter imaging using collimators on each of an array of detector elements. While such approaches give rise to detectors that are sensitive to the direction of x-ray propagation, the use of the collimators severely limits photon efficiency.
Alternative approaches for imaging systems have been developed that employ multiplexed measurements from shaped x-ray beams, such as a fan-shaped beam, to construct x-ray scatter images. Examples of such systems are disclosed in U.S. Pat. No. 7,583,783 entitled “X-ray Computer Tomograph and Method for Examining a Test Piece Using an x-ray Computer Tomograph.” Unfortunately, these approaches require multiple exposures and are relatively poorly conditioned for mathematical image estimation.
Backscatter x-ray systems detect a relatively small amount of radiation that reflects from the object and use it to form an image. These systems are particularly attractive for applications where less-destructive examination is required or where only one side of the target is available for examination. The backscatter pattern is dependent on the properties of the material being interrogated, and is good for imaging a wide range of materials. Backscatter x-ray imaging systems include full-body airport scanners, which are currently being used to detect hidden weapons, tools, liquids, narcotics, currency, etc. As with forward scatter, in backscatter systems that rely on collimators rather than coded apertures only a small percentage of the incident radiation is detected. Backscatter x-ray systems require high-power x-ray sources and/or high-sensitivity detectors in order to provide acceptable resolution and signal-to-noise ratio (SNR). Line-scan systems utilizing a fan beam of radiation to inspect an object and a segmented detector to measure radiation transmitted through the object are able to use a higher portion of available source flux; however, they are generally incapable of producing images from backscattered radiation.
Forward-scattering x-ray scatter imaging systems such as those described in U.S. Pat. No. 7,835,495, employ a primary beam of x-ray radiation that is scanned over an object while radiation from elastic (coherent) scattering is monitored by a fixed-position, energy-resolving detector. The detector is located at a small, fixed angle to the direction of propagation of the primary beam. Information about the crystallographic structure of the material of the scattering object is derived from the resultant scatter spectra. This information can then be compared to known scatter spectra in a library of materials of interest to determine if any such materials are included in the object being scanned.
Unfortunately, scanning x-ray systems capture only a small fraction of the radiation directed at the scanned object and are, therefore, highly inefficient. As a result, in order to produce an output signal having sufficiently high SNR, they require either x-ray sources capable of high power to increase the available radiation at the detector, or long exposure times. In either case, this exposes the scanned object to excessive amounts of x-ray radiation, which can be undesirable in many applications.
In addition, conventional x-ray imaging systems typically employ substantially monochromatic radiation or rely on energy discriminating detectors to improve resolution and signal quality. As a result, the total incident photon flux on the object of interest is limited.
Further, while conventional x-ray diffraction imaging approaches might be suitable for interrogating small-size (<1 cm2) areas, their data acquisition time makes it impractical to scan entire bags or parcels. The size limitation arises, in part, from the fact that while energy resolving detectors can discriminate x-ray diffraction orders from different wavelengths, they are quite expensive—particularly in array sizes necessary for high-speed, high-resolution imaging. Improved “fan beam” tomographic imaging systems offer some improvement in detection efficiency; however, these systems require expensive energy-resolving x-ray detectors and/or low-flux low-bandwidth x-ray sources. Still further, these systems are not well suited for scanning arbitrary objects whose composition can vary over a wide range since detection of a constituent material requires some advance knowledge or suspicion of the presence of that material so that its scattering “fingerprint” can be included in the material library.
Computational x-ray tomography, such as is described in U.S. Pat. No. 7,583,783, has been shown capable of producing images that are also based on measurements of the low-angle x-ray diffraction properties of an object. Such systems typically scan a “pencil beam” of x-ray radiation over a series of locations on the object and use computational processing-over many exposures to acquire a diffraction pattern for each scanned location. These diffraction patterns are used to reconstruct a series of images, which represent the coherent-scatter intensity at a series of scatter angles. Coherent-scatter cross-sections of the object can then be generated for each pixel from the sequence of images to develop a tomographic reconstruction of the object.
Like x-ray scatter-imaging systems, however, computed x-ray tomography systems do not efficiently utilize the x-ray energy directed at the scanned object. In addition, the need to develop the tomographic model of the object one cross-section at time leads to an undesirable space-time spectral trade-off.
There remains a need, therefore, for an improved imaging system that noninvasively ascertains the structural and molecular composition of three-dimensional objects at high speed and with relatively lower cost and complexity.